Ultrasound-guided obturator nerve block is utilized as a diagnostic and therapeutic maneuver in the evaluation and treatment of lower extremity pain thought to be mediated via the obturator nerve. The most common pain syndrome mediated via the obturator nerve is obturator neuropathy caused by trauma from pelvic fractures; surgical trauma, especially during total hip arthroplasty; or entrapment of the obturator nerve by tumor or by the fetal head during delivery. Patients with significant obturator neuropathy lose the ability to abduct and externally rotate the hip (Fig. 118.1). The patient will exhibit a typical gait abnormality that consists of an externally rotated foot. Wasting of the adductor muscles of the thigh and numbness of the distal medial thigh may also be identified.

Ultrasound-guided obturator nerve block can also be utilized to provide anesthesia for the lower extremity surgery when combined with lateral femoral cutaneous, femoral, sciatic nerve block, and/or lumbar plexus block. Ultrasound-guided obturator nerve block with local anesthetics can be employed as a diagnostic maneuver when performing differential neural blockade on an anatomic basis to determine if the patient’s lower extremity pain is subserved by the obturator nerve. If destruction of the obturator nerve is being contemplated, ultrasound-guided obturator nerve block with local anesthetic can provide prognostic information as to the extent of motor and sensory deficit the patient will experience following nerve destruction.

Obturator nerve block with local anesthetic may be used to palliate acute pain emergencies, including postoperative pain
relief while waiting for pharmacologic methods to become effective. Obturator nerve block with local anesthetic may also be used to block adductor spasm during transurethral prostate resections and to allow perineal care or urinary catheterization in patients who suffer from adductor spasm due to neurologic injury. Obturator nerve block with local anesthetic and steroid is also useful in the treatment of persistent hip pain when the pain is thought to be secondary to inflammation or entrapment of the obturator nerve. Destruction of the obturator nerve is occasionally indicated for the palliation of persistent hip and/or distal medial thigh pain that is mediated by the obturator nerve.

Electromyography can distinguish obturator nerve entrapment from lumbar plexopathy and lumbar radiculopathy. Plain radiographs of the hip and pelvis are indicated in all patients who present with obturator neuralgia to rule out occult bony pathology (see Fig. 118.1). Based on the patient’s clinical presentation, additional testing may be warranted, including a complete blood count, uric acid level, erythrocyte sedimentation rate, and antinuclear antibody testing. Magnetic resonance imaging of the lumbar spine and lumbar plexus and retroperitoneum is indicated if herniated disc, tumor, or hematoma is suspected. The injection technique described later serves as both a diagnostic and a therapeutic maneuver.

The obturator nerve is derived from the posterior branches of the L2, L3, and L4 nerve roots (Fig. 118.2). The nerve fibers enter the psoas muscle where they fuse together within the muscle body and leave the medial border of the psoas at the brim of the pelvis (Fig. 118.3). The nerve passes behind the common iliac arteries to run adjacent to the lateral wall of the pelvis where it joins the obturator artery and vein. Along with the obturator artery and vein, the obturator nerve enters the obturator canal to pass into the proximal thigh. At this point the nerve divides into an anterior branch, which provides sensory innervation to the hip joint, motor branches to the superficial hip adductors, and a cutaneous branch to the medial aspect of the distal thigh, and a posterior branch, which provides motor innervation to the deep hip adductors and an articular branch to the posterior knee joint (see Fig. 118.3).

Ultrasound-guided obturator nerve block can be carried out by placing the patient in the supine position with the arms resting comfortably across the patient’s chest (Fig. 118.4). A total of 14

mL of local anesthetic is drawn up in a 20-mL sterile syringe. If the painful condition being treated is thought to have an inflammatory component, 40 to 80 mg of depot steroid is added to the local anesthetic. The inguinal crease on the affected side is identified, and a linear high-frequency ultrasound transducer is placed in an oblique plane perpendicular with the inguinal ligament (Fig. 118.5). An ultrasound survey scan is obtained, and the iliacus muscle is identified with the femoral nerve lying between the muscle and the pulsatile femoral artery (Fig. 118.6). The femoral vein lies medial to the femoral artery and is easily compressible by pressure from the ultrasound transducer (Fig. 118.7). Color Doppler can be utilized to aid in the identification of the femoral artery and vein (Fig. 118.8). When these anatomic structures are clearly identified on oblique ultrasound scan, the ultrasound transducer is moved medially until the pectineus muscle, which looks like a “breaching whale,” is visualized (Figs. 118.9 and 118.10). Just medial to the medial aspect of the pectineus muscle lie the adductor longus, brevis, and
magnus muscles, which are stacked on top of one another like a double-decker sandwich (Fig. 118.11). The anterior branch of the obturator nerve lies in the fascial cleft between the adductor longus and brevis muscles, and the posterior branch of the obturator nerve lies between the fascial cleft between the adductor brevis and magnus muscles (Fig. 118.12). When the adductor muscles and the anterior and posterior branches of the obturator nerve are identified, the skin is prepped with anesthetic solution, and a 3½-inch, 22-gauge needle is advanced from the inferior border of the ultrasound transducer and advanced utilizing an out-of-plane approach with the trajectory being adjusted under real-time ultrasound guidance so that the needle passes through the adductor longus muscle and the needle tip rests in the fascial cleft between the adductor longus and brevis muscles in proximity to the anterior branch of the obturator nerve (Fig. 118.13). At that point, after careful aspiration, a small amount of solution is injected under real-time ultrasound imaging to utilize hydrodissection to reconfirm the position of the needle tip. Once the position of the needle tip is reconfirmed, after careful aspiration, an additional 6 mL of the solution is slowly injected under ultrasound guidance. There should be minimal resistance to injection. The needle is then withdrawn and redirected through both the adductor longus and brevis until the needle tip rests in the fascial cleft between the adductor brevis and magnus muscles in proximity to the posterior branch of the obturator nerve (see Fig. 118.13). At that point, after careful
aspiration, a small amount of solution is injected under realtime ultrasound imaging to utilize hydrodissection to reconfirm the position of the needle tip. Once the position of the needle tip is reconfirmed, after careful aspiration, an additional 6 mL of the solution is slowly injected under ultrasound guidance. There should be minimal resistance to injection. The needle is then removed, and a sterile pressure dressing and ice pack are placed at the injection site.